Recently, electrooptic TE.revreaction.TM mode conversion has been demonstrated in Ti-diffused lithium niobate waveguides. (See, for example, the article by R. C. Alferness entitled "Efficient, Waveguide Electrooptic TE.revreaction.TM Mode Converter/Waveguide Filter" Applied Physics Letters, 36, pages 513-515, Apr. 1, 1980.) Because of the large birefringence of lithium niobate, periodic coupling is required to achieve phasematching. This, in turn, makes the converter highly wavelength selective. As a result, such devices can be used as wavelength filters and, filters based upon these principles have been constructed. (See, for example, U.S. Pat. Nos. 3,877,782, and the 4,273,411.
However, it has been found that their response characteristics, as a function of wavelength, include spurious peaks at wavelengths above and below the desired wavelength. Efforts to reduce these extraneous peaks by weighting the coupling along the coupling interval have been partially successful, but significant sidelobes still remain.